1. Field of the Invention
The present invention relates to a method and apparatus for detecting errors for each of a plurality of virtual paths established along a designated section within a communication network, and more particularly to a method and apparatus for performing tandem connection maintenance (TCM) for each lower-order path, such as virtual container-1 (VC-1), in a synchronous digital hierarchy (SDH) transmission path.
2. Description of the Related Art
With recent progress in digital communication networks based on synchronous network design, the use of SDH apparatus has been increasing. When constructing a network using SDH apparatus, the provision of a TCM facility becomes an essential requirement.
When there is a service path extending over different network suppliers, the TCM facility is used to monitor network communication quality in the section served by each individual network supplier. TCM is performed, for example, by detecting reception errors in each virtual container (VC) at both ends of a virtual path established along a monitoring section, and by comparing the detection results obtained at both ends, thereby determining the presence or absence of new errors caused along the monitoring section and the number of such errors. More specifically, when the virtual container exiting from the monitoring section contains a certain number of errors, for example, if it is found that the virtual container entering the monitoring section contains the same number of errors, it is shown that the control section itself is free from errors. On the other hand, when the virtual container exiting from the monitoring section contains errors despite no errors being contained in the virtual container entering it, this means that there is some sort of fault in the monitoring section. In this way, when a fault occurs in a path established extending over different network elements served by different network suppliers, it is possible to distinguish the responsible network supplier from other suppliers.
To accomplish this processing, it is necessary to transmit the error detection result at one end of the path in the monitoring section to the other end either by using a path overhead of a VC being carried in the same direction as the transporting direction of the VC under inspection or by using a path overhead of a VC being carried in the opposite direction to the transporting direction of the VC under inspection. For higher-order paths such as the VC3/VC4 paths conforming to CCITT recommendations and the STS-1 path used in SONET under ANSI standards in North America, work is in progress for the standardization of TCM for transmission of detection results by a data link method using an overhead bit, conforming to the LAPD protocol.
However, for lower-order paths, since the number of paths accommodated in one transmission path is large (for example, one VC-4 contains 21 VC-12's), the same technique as used for higher-order paths would require extremely large circuitry, and is therefore impracticable. Under such circumstances, work on standardization has so far seen little progress.
On the other hand, in the lower-order path TCM method currently under study by CCITT, the Z6 byte of a lower-order path is assigned to TCM bits, with the more significant three bits defined as the incoming error count (IEC) and the less significant five bits as the tandem connection data channel, for transmission of the IEC. This method is simple compared to the technique used for higher-order paths.
However, with the lower-order path TCM method under study by CCITT, each time the Z6 byte is changed for transmission of IEC when performing TCM on a path, the result of BIP (Bit Interleave Parity)-2 indicating the parity calculation result also changes, requiring recalculation and reinsertion, and hence the problem of increased circuit complexity.